I haven't unlocked any return slots yet, but when I do then I'm not sure I even will put anything in them. Where's the fun in taking away all the neat gameplay options that Desktop Dungeons might happen to throw at you?

If I do though, it will probably be the Stone Signil. The total piety gained from it is low and requires wasting popcorn to gain, it doesn't convert for all that much afterwards, and even when I actually find it early my response is to save the gold for an item with an immediate effect to start earning bonus experience. Compared to how much easier and more effective it is to snag, use and convert the Agnostic's Collar should I turn out to need it ...

I can't imagine ever forbidding the Pendant of Health or Hero's Helm. Those are two of my most purchased items for a quick low-level kick, and they still help everyone a little by the end of the run.

Stone Sigil is absolutely awesome on deities who can otherwise be picky about gaining piety in some situations (EM) or have high penalties for killing certain types (Dracul, MA.) It's also nice if you're using Pactmaker's Warrior or Body pacts, since it blunts the cost of those decently.